The difference between 3M's Hookit and Hookit II Hook-N-Loop Interface Systems
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On the left hand side is ...

On the left hand side is the 3M 3" Hookit Backing Plate and just above it is the Meguiar's 3" Interface Pad, these two items are Hook-n-Loop Compatible. You must use Hookit discs with the Hookit Backing Plate. The Hookit system follows the industry standard for hook-n-loop interfaces.

On the right hand side is the 3M 3" Hookit II Backing Plate, the Meguiar's 3" Interface Pad will not work with this backing plate. You must use Hookit II discs with the Hookit II Backing Plate. The Hook-n-Loop is reversed on this 3M system.

3M 3" Hookit Backing Plate with "Hooks" on the face of the backing plate
(This is the industry standard system for hook-n-loop interface)

Here you can see the J-Hooks - Note there are different designs of hooks, not just the shape of a "J".

Note: If you've ever noticed over time that your pad and backing plate don't attach as firmly as when they were first new, it's because over time the "hooks" will tend to straighten out from use and thus won't hold the loops as tightly.